Adenovirus vectors have shown promise for vaccines, but one key hurdle is preexisting immunity to these viruses in humans. The VRC has been investigating the use of priming with either DNA or adenovirus vaccines and the use of alternate adenoviral serotypes or modified adenoviral vectors to overcome prior immunity. During the past year, the VRC has evaluated various serotypes, including non human primate adenoviral serotype vectors to analyze their ability to induce an immune response as compared to Ad5 and in the presence of Ad5 seropostivity. Several of these vectors are being further tested to evaluate their potential use in clinical trials.